If I understand the TSA's 3-1-1 policy in the US correctly, you are allowed to take two 3-ounce containers of a liquid on a plane, but not a single 6-ounce container of the same liquid. Is this really how the policy works? If so, what was the rationale for regulating carry-ons like this?

Interesting that the limit is 3 ounces (I'm assuming thats 3 US fluid ounces) which is 88mls. That's kind of annoying for the metric world as our limit is normally 100mls meaning all my containers are 12mls over size.
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StuartDec 25 '11 at 0:33

When I later looked at the TSA site that @nate-eldridge linked to, TSA said that 3.4 oz / 100 ml is adequate for a short trip. So knowing people really complain if they can't take anything, this is the smallest reasonable amount.
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mkennedyDec 26 '11 at 18:42

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I think the problem is that you're looking for logic in what many people call "security theater" -- the appearance that they are doing something, even if that thing is not effective. On a recent flight, I watched TSA discard a passenger's toothpaste because the tube was over the limit, but they allowed that same passenger to carry on two quarts of "prescription shampoo" without further inspection. And how did they know that the shampoo was prescription? The passenger said so. So, these rules mean that a half used 6 ounce tube of toothpaste is a hazard, but 64 ounces of shampoo is just fine.
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JohnnyDec 16 '14 at 18:17

3 Answers
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When you're making rules like this, simplicity and objectivity are vital. You don't want (whether you're a traveller, a supervisor of the security staff, or a person trying to prevent terrorism) a situation where security staff need to make decisions on their feet based on whether the passenger's story is good enough or any other kind of judgement call. Hmm, you have shampoo and conditioner, well yes your hair is quite long, you may pass. Oh my, you are almost completely bald, why would you need conditioner, we're confiscating this, and so on.

Travellers want to be confident their stuff won't be banned. Staff need to be able to do their jobs as quickly and consistently as possible. And the person who made the rules needs to be sure they will always be enforced as imagined. That's why you can't bring 50 ml of stuff in a 600 ml bottle. Reading the label and seeing that it says 600 ml is simple, unambiguous and consistent. It works even if the security staff are not that bright. And there's no need for a complicated appeal process. Someone else reads the label and says yup, 600 ml, this is a no-go. There's no argument about whether you have 80 ml left in the bottle or 120 ml left in the bottle. The rule is just about the bottle.

Your little bottles are each under 100ml and all fit in the baggie of freedom? You're free to go. Nobody stops to say hey, each of these bottles appears to have the very same stuff in it. That would require judgement. The volumes - the 100 ml and the dimensions of the baggie of freedom - have been chosen so that even if you brought 6 or 7 bottles (whatever can fit) of the maximum allowed size all of the same stuff, you wouldn't be able to do much damage. (And really, checking that they are all labelled as different things wouldn't do much - we can all make labels.) Also, if you used larger sub-bottles - say 200 ml - you could get more total volume into the baggie, so using small sub-bottles is another way of constraining the total volume.

Designing a very simple and consistent system makes the checking faster. It lets people bring shampoo, conditioner, lotion, toothpaste, shaving cream etc. And it was set up so that it still constrains the bad guys. Sort of. It's not that hard to circumvent, in reality, since there are exemptions for things that are labelled (eg saline solution, prescription medication) and tremendous trust in labels. But it's all tradeoffs, and this certainly makes people feel protected. It even has some effect. Nothing is 100%, but the liquid-reducing measures do mean that more conspirators would be required for most terror acts, and more conspirators means more chances to catch them before they even get to the airport.

According to the TSA, the limit is such that it would be impossible to use liquid/plastic explosives to take the plane down, while still allowing us to have our minimal amount of soap and shampoo for these 2-day business trips without having to check in any luggage.

Thats an interesting idea, would the X-RAY not pick it up? Originally the ban on liquids was to do with liquid bombs but I'm not sure how the current rules specifically came about. news.bbc.co.uk/1/hi/uk/8242479.stm
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StuartDec 25 '11 at 1:25

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Hmm, I can't imagine being able to conceal something in 6 ounces of liquid you couldn't conceal in two 3-ounce bottles.
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jrdiokoDec 25 '11 at 6:29